Certain automotive engines are equipped with a low pressure fuel injection system having an electrically energized injector which delivers fuel in timed pulses into the engine air induction passage above the throttle. Such a fuel injection system is currently known as a throttle body injection system or a TBI system. In a system of that nature, fuel flow is controlled by energizing the injector at regular intervals and varying the duration of the fuel delivery pulses: when increased fuel delivery is desired, the injector is energized for a longer period of time to increase the duration of the fuel delivery pulse; when decreased fuel delivery is desired, the injector is energized for a shorter period of time to decrease the duration of the fuel delivery pulse.
It has been recognized, of course, that variations in the pressure of the fuel supplied to the injector also affect fuel delivery by the injector. Accordingly, in order to provide predictable and repeatable fuel delivery by the injector in response to the duration of the fuel delivery pulses, a fuel pressure regulator is employed to maintain a constant fuel supply pressure. The fuel is supplied to the injector by a pump, and the pump supplies more fuel than is required by the injector. The excess fuel is directed through a fuel pressure regulator represented, for example, by U.S. Pat. No. 3,511,270. The pressure regulator has a diaphragm which balances the pressure of the fuel supplied to the injector with the bias of a pressure regulator spring. If the pressure of the fuel supplied to the injector is less than the spring bias, the diaphragm positions a valve to shut off the excess fuel flow and thus increase the fuel pressure at the injector; if the pressure of the fuel supplied to the injector exceeds the spring bias, the diaphragm retracts the valve to discharge the excess fuel and thus reduce the fuel pressure at the injector.
It is evident, therefore, that the fuel pressure at the injector is determined by the bias of the pressure regulator spring, and the desired fuel pressure is established by adjusting the bias of the spring.
The fuel flow orifice in an electrically energized injector must be large enough to deliver the maximum fuel flow required by the engine when the injector is continuously energized. Preferably, the fuel flow orifice is sized to deliver the maximum quantity of fuel required for any pulse when the injector is energized for a period of time which is 90% of the pulse-to-pulse interval--that is, the maximum quantity of fuel is delivered when the injector is energized with a 90% duty cycle. Then when a lesser quantity of fuel is required, the injector is energized with a proportionally lesser duty cycle. Thus it will be appreciated that the size of the fuel flow orifice establishes the duty cycle which will deliver the minimum quantity of fuel required for a pulse occurring during an operating condition such as engine deceleration. For example, an increase in the size of the fuel flow orifice necessary to increase the maximum quantity of fuel delivered by the injector is accompanied by a decrease in the duty cycle employed to deliver a specified minimum quantity of fuel in each pulse.
However, an electrically energized injector must be energized for at least a minimum period of time to deliver a predictable and repeatable quantity of fuel in each pulse. Clearly care must be exercised to avoid a situation in which the duty cycle employed to deliver the minimum quantity of fuel produces a pulse duration which is less than the minimum period of time required to deliver a predictable and repeatable quantity of fuel.
Of course, the fuel flow orifice could be sized so that the minimum quantity of fuel required for any pulse is delivered in the minimum period of time required for a predictable and repeatable fuel pulse. However, such a calibration would limit the maximum quantity of fuel which could be delivered in the maximum pulse duration. Thus the fuel flow authority of prior fuel injection systems employing electrically energized injectors has been limited by the size of the fuel flow orifice and by the minimum pulse duration.